1. Importance of Placement Accuracy
Thermal-inkjet printing is based on accurate ballistic delivery of small ink droplets to exact locations on paper or some other printing medium. Ordinarily the droplet placement is with respect to a grid of specified resolution, most common grids nowadays being 12-by-12 or 24-by-24 dots per millimeter (300-by-300 or 600-by-600 dpi). Other possibilities are continuously being considered.
One key requirement for sharp, high-quality images is accuracy of the droplet placement. Drop-placement error (DPE) causes line discontinuity and roughness--especially important in plotters used primarily for computer-aided design (CAD)--as well as banding and color inconsistencies that are significant in printers mainly used to reproduce graphics or photos.
2. Previously Recognized Error Sources, and Solutions
There are several contributors to droplet-placement inaccuracies. Some of these arise in the printhead, and others in the printer mechanism proper; inaccuracies can occur along the scan-axis or paper-axis direction. Some inaccuracies are systematic, while others follow random patterns.
The previously mentioned Majette patent is representative of earlier innovations in encoder subsystems that enable basic determination and servocontrol of printhead position and speed. The Raskin patent teaches how to operate the timing of bidirectionally scanning systems to provide consistent dot placement independent of scanning direction.
The Cobbs and Sievert patents address a still more sophisticated problem, namely control of the mutual alignment of multiple printheads operating on a common scanning carriage. That challenge is met by printing and reading test patterns, to determine the mechanical relationship between the heads on the carriage--and then by, in effect, shifting the operational nozzle arrays on certain of the pens to obtain alignment within specifications.
To facilitate the shifting process, heads are provided with a few extra nozzles at each end, so that the shift is reduced to merely a selection and renaming process. The patents to Cobbs and to Sievert make use of relatively small test patterns automatically printed, and then automatically read.
Other reported efforts make use of laser-based measurements for interpen alignment. This approach likewise is based on measurements taken in a limited-width portion of the printer image space.
3. Newly Discovered Error Mode
Despite these advances, residual errors in interhead alignment have been detected in a current generation of printer/plotters. These errors have an adverse effect on print quality, most conspicuously taking the form of cyan-to-black misalignment in certain portions of standard test images--for instance, particularly where a cyan background appears at one side of a black area fill.
Appearance of these residual errors has been markedly erratic--not arising in every prototype unit but only some units, and also not consistent in all parts of the printed images but rather only within some regions. Furthermore these errors are more severe for some printheads (i.e., certain colors) than others.
On most papers the error appears where vertical lines change color from black to cyan. Also, in plots containing black area fills adjacent to green or violet areas, a certain yellow halo (when green) or magenta halo (when violet) can be seen on misalignments of two pixel columns. In addition these errors are believed to cause higher graininess, mainly in gray area fills.
Awareness of this peculiarity arose in late phases of a product development. As far as we know, no previous worker in this field has attempted to develop an understanding of these mysterious and stubbornly persistent error residuals.
Not only the correction of these defects but also a first recognition of their basic character has only now been revealed. Accordingly a description of the source of these errors is not properly a part of this background section in the present document, and therefore is reserved for a following section summarizing the invention.
A somewhat analogous or related problem of bringing pen-to-paper distance within specifications is treated in the Maher, Castano and Boleda applications mentioned earlier.
4. Conclusion
Small and seemingly erratic dot-placement errors have heretofore impeded achievement of uniformly excellent inkjet printing. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.